Currency detectors



June 23, 1964 B. S. cAHlLL CURRENCY DETECTORS Filed Oct. 19. 6

IN VEN TOR. BERNARD S. CAH\LL United States Patent 3,138,738 CURRENCY DETECTORS Bernard S. Cahill, Kirkwood, Mo-., assignor to National Rejectors, Inc., St. Louis, Mo., a corporation of Missouri Filed Oct. 19, 1960, Ser. No. 63,547 14 Claims. (Cl. 315- 168) This invention relates to improvements in currency detectors. More particularly, this invention relates to improvements in electronic circuits for currency detectors.

It is therefore an object of the present invention to provide an improved electronic circuit for currency detectors.

This invention is an improvement on the first of the electronic circuits shown and described in Smith et al. application Ser. No. 849,066 for Currency Detectors which was filed October 27, 1959. In the currency detector dis closed in the said Smith et al. application, the magnetic face of an inserted bill was held in engagement with the air gap of a magnetic head while that bill was moved past that magnetic head at a predetermined speed. The mag netic ink in the grid lines of the two portions of the portrait background, which =are disposed at opposite sides of the portrait on that bill, coaoted with that magnetic head to provide two groups of voltage variations; and those two groups of voltage variations were then amplified, limited and fed to resonant circuit. If the bill was authentic, a controlled cumulative voltage growth was experienced which triggered a threshold device such as a thyratron.

In the said electronic circuit disclosed in the said Smith et al. application, the threshold device was driven with AC. voltage; and such voltage is beneficial because it facilitates quenching of the threshold device after that device has been fired in response to the first controlled cumulative voltage growth. Such quenching is necessary because the threshold device must ready itself for the second controlled cumulative voltage growth which will be experienced by the resonant circuit. The use of AC. voltage to drive the threshold device would be very acceptable if the portrait backgrounds of all bills had large numbers of grid lines at the opposite sides of the portraits, and if the portraits of all bills were free of lines that gave responses which were generally similar to the responses given by the grid lines of the portrait background. However, the number of grid lines in the portrait background at one side of the portrait of George Washington on the one dollar bill is quite limited, and certain portions of the beard of Abraham Lincoln on the five dollar bill and certain portions of the hair of Benjamin Franklin on the one hundred dollar bill give responses which are generally similar to the responses given by the grid lines of the portrait background on the one dollar bill. As a result, the use of AC. voltage to drive the threshold device in the said Smith et al. application could lead to the rejection of some authentic one dollar bills and could lead to the acceptance of an authentic five dollar bill or an authentic one hundred dollar bill as a one dollar bill.

Specifically, the controlled cumulative voltage growth experienced by the resonant circuit of the tuned amplifier of the said first electronic circuit of the said Smith et al. application requires a minimum of six grid lines. Yet, the hair of George Washington is so close to one of the sides of the portrait frame on the one dollar bill that only eleven, twelve or thirteen grid lines can engage the air gap of a magnetic head that is set in register with the hair of George Washington. If the bill is being moved at such a rate that the grid lines pass the air gap of the magnetic head at the rate of a thousand per second, eleven grid lines will pass that air gap in a period of eleven milliseconds and thirteen grid lines will pass that air gap in a period of thirteen milliseconds. Even the thirteen milliseconds period is shorter than the duration of a single cycle of sixty cycle AG, and the eleven milliseconds period is even shorter; because one cycle of sixty cycle A.C. requires sixteen and six tenths milliseconds. To be able to trigger the threshold device the grid lines must pass the air gap of the magnetic head during the positive-going portion of the sixty cycle A.C., because that threshold device has a negative bias that must be overcome before that device can be triggered. In fact, the grid lines must pass the air gap of the magnetic head during the central section of the positive-going portion of the sixty cycle A.C., because the amplitude of that portion must rise to a value which will enable it to reduce the bias on the threshold device to the point where that device will fire. All of this means that where the threshold device is driven by AC. voltage, six of the eleven, twelve or thirteen grid lines must engage the air gap of the magnetic head during the central section of the positive-going portion of the AC; and, in actual practice, this did not happen every time. As a result, authentic one dollar bills could be, and were, rejected; and hence the use of AC. voltage to drive the threshold device of a currency detector could, and did, lead to the rejection of authentic one dollar bills.

The use of AC. voltage to trigger the threshold device of a currency detector can also lead to the acceptance of an authentic five dollar bill or an authentic one hundred dollar bill as an authentic one dollar bill. Specifically, the AC. provides such prompt quenching of the threshold device that the said device can respond to controlled cumulative voltage growth which can be experienced by the resonant circuit as the beard of Abraham Lincoln in the five dollar bill or the hair of Benjamin Franklin on the one hundred dollar bill passes the air gap of the magnetic head. In the case of Abraham Lincolns beard, noise or other transient voltage variations could produce the first controlled cumulative voltage growth and then that heard could produce the other controlled cumulative volt age growth. In the case of Benjamin Franklins hair, that hair could produce both of the required controlled cumulative voltage growths. The overall result is that the AC. voltage driven threshold device could, and did, occasionally accept five dollar and one hundred dollar billsas one dollar bills.

The present invention avoids undue rejections of authentic one dollar bills and avoids the acceptance of authentic five dollar bills and authentic one hundred dollar bills as authentic one dollar bills by providing a DC. driven threshold device which is driven through a circuit breaker. That circuit breaker has a time-delay circuit associated with it; and, as a result, that circuit breaker will remain open to provide a blanking period for the threshold device. In this way, that circuit breaker will normally remain closed to drive the threshold device but will respond to the energization ozEthat device to open the circuit of that device and will then hold that circuit open for a predetermined period of time to provide a blanking period for that device. It is therefore an object of the present invention to provide a DC. driven threshold device which is driven through a circuit breaker that has a time-delay circuit associated with it.

Other and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying description a preferred embodiment of the present invention is shown and described but it is to be understood that the drawing and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will be defined by the appended claims.

The drawing is a schematic diagram of one preferred form of electronic circuit that is made in accordance with the principles and teachings of the present invention.

Referring to the drawing in detail, the numeral 940 denotes the primary winding of a transformer which will be suitably connected to a source of alternating current. The numeral 944 denotes the secondary winding of that transformer which provides the voltages for the filaments of the various tubes used in the electronic circuit of the present invention. A potentiometer 948 is connected across the terminals of the secondary winding 944, and the center tap of that potentiometer is connected to ground as at 949. The numeral 946 denotes the high voltage secondary winding of the transformer; and the numeral 942 denotes an electrostatic shield for that transformer which minimizes stray coupling to the secondary winding 944.

One terminal of the secondary winding 946 is connected directly to one terminal of a voltage regulator tube 962 and to one terminal of a neon tube 976. The other terminal of that secondary winding is connected to the plate of that voltage regulator tube through a currentlimiting resistor 956, a diode 954, junction 961, a resistor 958, junction 963, and a resistor 964, and is connected to the other terminal of the tube 976 by resistor 956, a diode 966, a resistor 970, and a resistor 974. The one terminal of the secondary winding 946 is grounded; and a capacitor 960 is connected between that grounded terminal and the junction 961 between diode 954 and resistor 958. A capacitor 959 is connected between junction 963 and the grounded terminal of the secondary winding 946. The capacitors 959 and 960 coact with the resistor 958 to filter the rectified wave form provided by the secondary winding 946 and the diode 954. A capacitor 972 is connected between the grounded terminal of secondary winding 946 and the junction between diode 966 and resistor 970, and a capacitor 973 is connected between the grounded terminal and the junction between the resistors 970 and 974. The capacitors 972 and 973 coact with the resistor 970 to filter the rectified wave form provided by the secondary winding 946 and the diode 966. The resistor 956 acts to limit the values of the current which can flow through the diodes 954 and 966. A resistor 990, a potentiometer 992 and a resistor 996 are connected in series intermediate the lower terminal of the neon tube 976 and the grounded terminal of the secondary winding 946. Those resistors and that potentiometer provide a source of negative bias voltage.

The numeral 1008 denotes two terminals that can be connected to the two terminals of the magnetic head 1100 of a currency detector. A resistor 1010 and a capacitor 1012 are connected across those terminals, and hence will be in parallel with the coils of the magnetic head 1100. A resistor 1016 and a resistor 1047 are connected in series intermediate the plate of the voltage regulating tube 962 and the junction of capacitor 1012 and resistor 1010; and therefore direct current will flow through those resistors and through the coils of the magnetic head 1100.

A capacitor 1018 and a resistor 1020 couple the voltage variations from the magnetic head 1100 to the grid of a triode 1022. That triode is, for convenience, a part of a duo-triode. That triode is connected to the plate of the voltage regulator tube 962 by a resistor 1024 and resistor 1047. A capacitor 1026 and a resistor 1028 couple the plate of the triode 1022 to the grid of a triode 1030. For convenience, that triode is the other half of the duotriode of which the triode 1022 is a part. The plate of the triode 1030 is connected to the plate of the voltage regulator tube 962 by a resistor 1032 and the resistor 1047. j The plate of the triode 1030 is coupled to the grid of a triode 1038 by a capacitor 1034 and a resistor 1036. For convenience, the triode 1038 can be one half of a duotriode. A resistor 1035 is connected between the grid of triode 1038 and the junction of capacitor 1034 and resistor 1036. The plate of the triode 1038 is connected to the plate of the voltage regulator tube 962 by a resistor 1040 and resistor 1047. ,The plate of the triode 1038 is coupled to the grid of a triode 1046 by a capacitor 1042 and aresistor 1044. For convenience, the triode 1046 can be the other half of the duo-triode of which the triode 1038 is a part. A capacitor 1045 has one terminal thereof connected intermediate the resistors 1040 and 1047 and has the other terminal thereof grounded. That capacitor coacts with the resistor 1047 to provide a de-coupling action which decreases the effect which power supply impedanccs and which transients in the power supply could have on the low level input stages of the electronic circuit shown by the drawing. The plate of the triode 1046 is connected to the plate of the voltage regulator tube 962 by a resistor 1048.

The plate of the triode 1046 is connected to the junction of two capacitors 1050 and 1052, and those capacitors coact with the inductor 1054 to constitute a series resonant circuit. That resonant circuit will be tuned to the frequency at which the grid lines in the portrait background of an authentic bill Will be moved past the magnetic head of a bill-transporting device. That series resonant circuit is coupled to the grid of a thyratron 1064 by capacitor 1058 and resistors 1060 and 996 and the upper portion of potentiometer 992. A resistor 1056 is connected intermediate capacitor 1058 and resistor 1060, and a resistor 1062 is connected between the grid of thyratron 1064 and the junction between resistors 1056 and 1060. The resistor 996 and the upper portion of potentiometer 992 will establish the value of the negative bias for the grid of the thyratron 1064, and that value will be such as to normally keep that thyrarton from conducting current.

The plate of the thyratron 1064 is connected to the junction 963 by a coil 428 of a relay and by the normallyclosed contacts 1067 of that relay. Those relay contacts constitute a circuit breaker in the circuit of the coil 428 of the relay. The coil 428 also controls a movable contact that normally engages the fixed contact 438 but that can respond to energization of the coil 428 to move down into engagement with the fixed contact 430. The relay coil 428 and the fixed contacts 438 and 440 perform all the functions performed by the identically-numbered parts in the said Smith et al. application. A capacitor 1066 is connected in parallel with the relay coil 428 and coacts with that coil to constitute a time-delay circuit.

The electronic circuit provided by the present invention is very similar to the said first electronic circuit in the said Smith et al. application; and most of the components of the two electronic circuits are identical and bear the same numbers. One difference between the two circuits is that in the accompanying drawing the over-level control of the Smith et al. application has not been shown; but this has been done merely to simplify this description of the present invention. Another difference is that the current-limiting resistor 956 of the present invention has been made to perform the function of the two separate current-limiting resistors shown in the said electronic circuit of the said Smith et a1. application. Also, the resistor 996 has been made to perform the function of two resistors in the said electronic circuit of the said Smith et al. application. In addition, the resistor 1035 has been interposed between the junction of capacitor 1034 and resistor 1036 and the grid of triode 1038. Further, the de-coupling capacitor 1045 and resistor 1047 have been added.

The operation of the electronic circuit of the present invention is similar to the operation of the said first electronic circuit in the said Smith et al. application, in that the triode 1022 and the triode 1030 provide amplification of the voltage variations supplied to the terminals 1008. Also, the triodes 1038 and 1046 provide a limiting action; but the resistor 1035 enables the triode 1038 to start limiting at lower amplitudes than could be corresponding triode in the said Smith et al. application. Such limiting prevents noise pulses from puting a charge on the coupling condenser 1034 and thereby blocking the triod The triode 1046 suplies quantums of energy which have values below a predetermined maximum value and which generate oscillations in the tuned circuit constituted by capacitors 1050 and 1052 and by inductor 1054. The voltage across the inductor 1054 is applied between the cathode and grid of the thyratron 1064; and when the voltage across the inductor 1054 reaches a predetermined value that thyratron will fire.

A principal difference between the electronic circuit of the present invention and the said first electronic circuit of the said Smith et al. application is that the voltage at the plate of the thyratron 1064 in the present circuit is a D.C. Voltage whereas the voltage at the plate of the corresponding thyratron in the said first electronic circuit of the said Smith et a1. application was an A.C. voltage. The fact that the voltage at the plate of the thyratron 1064 in the present circuit is a DC voltage means that there need not be registration between the controlled cumulative voltage growth in the series resonant circuit and an alternating voltage applied to the plate of the thyratron 1064. Consequently, the very instant the voltage across the inductor 1054 attains the required value, the thyratron 1064 will fire. As a result, if the portrait background of an inserted authentic bill has grid lines of the required spacing,those grid lines will fire the thyratron 1064 irrespective" of the alternations of the voltage in the secondary winding 946.

The firing of the thyratron 1064 will energize the coil 428 of the relay, and that coil will open the normallyclosed circuit breaker 1067. That relay coil will also move the movable contact away from the fixed contact 438 and into engagement with the fixed contact 440. Those fixed contacts 438 and 440 will be part of a relay chain comparable to the first relay chain disclosed in the said Smith et a1. application; and, as a result, the firing of the thyratron 1064' will provide a validating signal.

The energization of the relay coil 428 will also provide quenching of the thyratron 1064; but it will provide a blanking period after it quenches that thyratron. As a result, the thyratron 1064 will be able to respond to voltage variations generated by grid lines at the first side of the portrait background but will not respond to further voltage Variations generated by further grid lines at the first side of the portrait background and will not respond to voltage variations generated by lines in the portrait itself. This desirable result is accomplished by the coaction of the capacitor 1066 with the coil 428 of the relay. Specifically, as the thyratron 1064 fires, it will act as a substantial short circuit to ground and will thus draw a large amount of current from the capacitor 1066 and will also cause current to pass through the relay coil 428. The current passing through that coil will enable that coil to open the circuit breaker 1067 and to move the movable contact down into engagement with the fixed contact 440. The breaking of the circuit by the opening of the circuit breaker 1067 will not lead to the immediate de-energization ofthe coil 428 because the capacitor 1066 will temporarily cause current to flow through that coil as the capacitor restores its electron balance; and that flow of current will be great enough to hold the coil 428 energized sufficiently to hold the contacts 1067 open and to hold the movable contact in engagement with the fixed contact 440'.

The period of time during which the capacitor 1066 will hold the relay coil 428 energized will be determined by thetime constant of that capacitor and of that coil. At the conclusion of that period of time, the relay coil 428 will become de-energized and the contacts 1067 will reclose and thereby re-set the circuits to the plate of the thyratron 1064. By this time, the vertical grid lines in the first half of the background of the portrait and the lines in that portrait itself will have passed by the air gap of the magnetic head; and that air gap will be sensing the grid lines in the other half of the portrait background. This means that any voltage variations that could be generated by Abraham Lincolns beard or by Benjamin Franklins hair will be eifectively blanked out and can not cause 6 the acceptance, respectively, of a five dollar bill or a one hundred dollar bill as a one dollar bill.

Whereas the drawing and accompanying description have shown and described a preferred embodiment of the present invention, it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

What I claim is:

1. In an electronic circuit for a currency detector which responds to a predetermined minimum number of voltage variations, that are generated as one side of the portrait background of an authentic bill is moved past the air gap of a magnetic head, to produce a first controlled cumulative voltage growth to fire a thyratron, which subsequently responds to the movement of the portrait of said bill past said air gap of said magnetic head to de-energize said thyratron, and which thereafter responds to a second predetermined minimum number of voltage variations, that are generated as the other side of the portrait background of said bill is moved past said air gap of said magnetic head, to produce a second controlled cumulative voltage growth to again fire said thyratron, the improvement which comprises a resonant circuit that receives voltage variations generated in said magnetic head and that is connected to the input of said thyratron, and a relay coil and a circuit breaker that are connected in series and that are adapted to normally connect the plate of said thyratron to a source of D.C. driving voltage, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can provide a cumulative voltage growth capable of firing said thyratron, said D.C. driving voltage providing a substantially constant driving voltage for said thyratron and thereby providing firing of said thyratron by the first said controlled cumulative voltage growth as soon as said growth attains a predetermined value and also providing firing of said thyratron again by said second controlled cumulative voltage growth as soon as said second growth attains said predetermined value, said relay coil responding to the firing of said thyratron to open said circuit breaker and thereby de-e'nergize said relay coil and quench said thyratron, and a capacitor that is connected in parallel with said relay coilto maintain said relay coil energized for a short period of time and thereby delay the re-closing of said circuit breaker, said period of time being comparable to the time required to move said portrait of said bill past said air gap of said magnetic head and thereby provide a blanking action while said portrait of said bill is being moved past said airgap of said magnetic head, said resonant circuit and said capacitor c'oacting to permit said thyratron to fire just once per side of said portrait background.

2. In an electronic circuit fora currency detector which responds to a predetermined minimum number of voltage variations, that are generated as one side of the portrait background of an authentic bill is moved past the air gap of a magnetic head, to produce a first controlled cumulative voltage growth tofire a thyratron, which subsequently responds to the movement of the portrait of said bill past said air gap of said magnetic head to de-energize said thyratron, and which thereafter responds to a second predetermined minimum number of voltage variations, that are generated as the other side of the portrait background of said bill is moved past said air gap of said magnetic head, to produce a second controlled cumulative voltage growth to again fire said thyratron, the improvement which comprises a resonant circuit that receives voltage variations generated in said magnetic head and that is connected to the input of said thyratron, and a relay coil and a circuit breaker that are connected in series and that are adapted to normally connect the plate of said thyratron to a source of D.C. driving voltage, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can provide a cumulative voltage growth capable of r i i r 7 firing said thyratron, said D.C. driving voltage providing a substantially constant driving voltage for said thyratron and thereby provide firing of said thyratron by the first said controlled comulative voltage growth as soon as said growth attains a predetermined value and also providing firing of said thyratron again by said second controlled cumulative voltage growth as soon as said second growth attains said predetermined value, said-relay coil responding to the firing of said thyratron to open said circuit breaker and thereby de-energize said relay coil and quench said thyratron, and a capacitor that can maintain said relay coil energized for ashort period of time and thereby delay the re-closing of said circuit breaker and thereby provide a blanking action, said resonant circuit and said capacitor coacting to permit said thyratron to fire just once per side of said portrait background.

3. In an electronic circuit for a currency detector which responds to a predetermined minimum number of voltage variations, that are generated as one side of the portrait background of an authentic bill is moved past the air gap of a magnetic head, to produce a first controlled cumulative voltage growth to fire a thyratron, which subsequently responds to the movement of the portrait of said bill past said air gap of said magnetic head to de-energize said thyratron, and which thereafter responds to a second predetermined minimum number of voltage variations, that are generated as the other side of the portrait background of said bill is moved past said air gap of said magnetic head, to produce a second controlled cumulative voltage growth to again fire said thyratron, the improvement which comprises a resonant circuit that receives voltage variations generated in said magnetic head and that is connected to the input of said thyratron, and a relay coil and a circuit breaker that are connected in series and that are adapted to normally connect the plate of said thyratron to a source of D.C. driving voltage, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can provide a cumulative voltage growth capable of firing said thyratron, said D.C. driving voltage providing a substantially constant driving voltage for said thyratron and thereby providing firing of said thyratron by the first said controlled cumulative voltage growth as soon as said growth attains a predetermined value and also providing firing of said thyratron again by said second controlled cumulative voltage growth as soon as said second growth attains said predetermined value, said relay coil responding to the firing of said thyratron to open said circuit breaker and thereby de-energize said relay coil and quench said thyratron and to hold said thyratron quenched for a predetermined minimum length of time, said resonant circuit coacting with said relay to permit said thyratron to fire just once per side of said portrait background.

4. In an electronic circuit for a currency detector which responds to a predetermined minimum number of voltage variations, that are generated as one side of the portrait background of an authentic bill is moved past the air gap of a magnetic head, to produce a first controlled cumulative voltage growth to fire a thyratron, which subsequently responds to the movement of the portrait of said bill past said air gap of said magnetic head to de-energize said thyratron, and which thereafter responds to a second predetermined minimum number of voltage variations, that are generated as the other side of the portrait background of said bill is moved past said air gap of said magnetic head, to produce a second controlled cumulative voltage growth to again fire said thyratron, the improvement which comprises a resonant circuit that receives voltage variations generated in said magnetic head and that is connected to the input of said thyratron, and a circuit breaker that is adapted to normally connect the plate of said thyratron to a source of D.C. driving voltage, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can provide a cumulative voltage growth capable of firing said thyratron, said circuit breaker responding to the firing of said thyratron to quench said thyratron, and a time delay element that holds said circuit breaker open for a short period of time and thereby provides ablanking action, said resonant circuit coacting with said time delay element to permit said thyratron to fire just once per side of said portrait background.

5. In an electronic circuit for a currency detector which responds to a predetermined minimum number of voltage variations, that are generated as one side of the portrait background of an authentic bill is moved past the air gap of a magnetic head, to produce a first controlled cumula-. tive voltage growth to fire a thyratron, which subsequently responds to the movement of the portrait of said bill past said air gap of said magnetic head to de-energize said thyratron, and which thereafter responds to a second predetermined minimum number of voltage variations, that are generated as the other side of the portrait background of said bill is moved past said air gap of said magnetic head, to produce a second controlled cumulative voltage growth to again fire said thyratron, the improve ment which comprises a resonant circuit that receives voltage variations generated in said magnetic head and.

that is connected to the input of said thyratron, and a relay coil and a circuit breaker that are connected in series and that are adapted to normally connect the plate of said thyratron to a source of D.C. driving voltage, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can provide a cumulative growth capable of firing said thyratron, said relay coil responding to the firing of said thyratron to open said circuit breaker and thereby de-energize said relay coil and quench said thyratron, and a capacitor that is connected in parallel with said relay coil to maintain said relay coil energized for a short period of time and thereby delay the re-closing of said circuit breaker and thereby provide a blanking action, said resonant circuit and said capacitor coacting to permit said thyratron to fire just once per side of said portrait background.

6. In an electronic circuit for a currency detector which responds to voltage variations due to an authentic bill to produce a first controlled cumulative voltage growth to actuate a threshold device, which subsequently de-energizes said threshold device, and which thereafter responds to further voltage variations due to said bill to produce a second controlled cumulative voltage growth to again actuate said threshold device, the improvement which comprises a resonant circuit that receives voltage variations and that is connected to the input of said threshold device, a D.C. driving voltage for said threshold device, a relay coil and a circuit breaker that are connected in series and that are adapted to normally connect said threshold device to said D.C. driving voltage, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can actuate said threshold device, said relay coil responding to the actuation of said threshold device to open said circuit breaker and thereby de-energize said relay coil and de-energize said threshold device, and a capacitor that is connected in parallel with said relay coil to maintain said relay coil energized for a short period of time and thereby delay the re-closing of said circuit breaker and thereby provide a blanking action, said resonant circuit and said capacitor coacting to provide a predetermined minimum length of time between the receipt of the first voltage variation and the re-closing of said circuit breaker.

7. In an electronic circuit for a currency detector which responds to voltage variations due to an authentic bill to produce a first controlled cumulative voltage growth to actuate a threshold device, which subsequently de-v energizes said threshold device, and which thereafter responds to further voltage variations due to said bill to produce a second controlled cumulative voltage growth to again actuate said threshold device, the improvement which comprises a resonant circuit that receives voltage variations and that is connected to the input of said threshold device, a DC. driving voltage for said threshold device, a relay coil and a circuit breaker that are connected in series and that are adapted to normally connect said threshold device to said D.C. driving voltage, said resonant circuit requiringa predetermined minimum number of said voltage variations at its resonant frequency before it can actuate said threshold device, said relay coil responding to the actuation of said threshold device to open said circuit breaker and thereby deenergize said relay coil and de-energize said threshold device, said resonant circuit and said relay coil coacting to provide a predetermined minimum length of time between the receipt of the first voltage variation and the re-closing of said circuit breaker.

8. In an electronic circuit for a currency detector which responds to voltage variations due to an authentic bill to produce a first controlled cumulative voltage growth to actuate a threshold device, which subsequently deenergizes said threshold device, and which thereafter responds to further voltage variations due to said bill to produce a second controlled cumulative voltage growth to again actuate said threshold device, the improvement which comprises a resonant circuit that receives voltage variations and that is connected to the input of said threshold device, a DC. driving voltage for said threshold device, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can actuate said threshold device, a circuit breaker that is adapted to normally connect said threshold device to said D.C. driving voltage, said circuit breaker responding to the actuation of said threshold device to de-energize said threshold device, and a time delay element that holds said circuit breaker open for a short period of time and thereby provides a blanking action, said resonant circuit and said time delay element coacting to provide a predetermined minimum length of time between the receipt of the first voltage variation and the re-closing of said circuit breaker.

9. In an electronic circuit for a currency detector which responds to voltage variations due to an authentic bill to produce a first controlled cumulative voltage growth to actuate a threshold device, which subsequently deenergizes said threshold device, and which thereafter responds to further voltage variations due to said bill to produce a second controlled cumulative voltage growth to again actuate said threshold device, the improvement which comprises a resonant circuit that receives voltage variations and that is connected to the input of said threshold device, a DC. driving voltage for said threshold device, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can actuate said threshold device, a circuit breaker that is adapted to normally connect said threshold device to said D.C. driving voltage, said circuit breaker responding to the actuation of said threshold device to de-energize said threshold device, said resonant circuit and said circuit breaker coacting to provide a predetermined minimum length of time between the receipt of the first voltage variation and the re-closing of said circuit breaker.

10. In an electronic circuit for a currency detector which responds to a predetermined minimum number of voltage variations, that are generated as one side of the portrait background of an authentic bill is moved past the air gap of a magnetic head, to produce a first controlled cumulative voltage growth to fire a thyratron, which subsequently responds to the movement of the portrait of said bill past said air gap of said magnetic head to de-energize said thyratron, and which thereafter responds to a second predetermined minimum number of voltage variations, that are generated as the other side of the portrait background of said bill is moved past said air gap of said magnetic head, to produce a second controlled cumulative voltage growth toagain 'fire said thyratron, the improvement which comprises a resonant circuit that receives voltage variations generated in said magnetic head and that is connected to the input of said thyratron, and a DC. driving voltage for said thyratron, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can provide a cumulative voltage growth capable of firing said thyratron, said D.C. driving voltage providing a substantially constant driving voltage for said thyratron and thereby providing firing of said thyratron by the first said controlled cumulative voltage growth as soon as said growth attains a predetermined value and also providing firing of said thyratron again by said second controlled cumulative voltage growth assoon as saidsecond growth attains said predetermined value.

11. In an electronic circuit for a currency detector which responds to voltage variations due to an authentic bill to actuate a threshold device, which subsequently deenergizes said threshold device, and which responds to further voltage variations due to said bill to again actuate said threshold device, the improvement which comprises a resonant circuit that receives voltage variations and that is connected to the input of said threshold device, a source of driving voltage for said threshold device, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can actuate said threshold device, a circuit breaker that is adapted to normally connect said threshold device to said source of driving voltage, said circuit breaker responding to the actuation of said threshold device to deenergize said threshold device, and a time delay element that holds said circuit breaker open for a short period of time and thereby provides a blanking action, said resonant circuit and said time delay element coacting to provide a predetermined minimum length of time between the receipt of the first voltage variation and the re-closing of said circuit breaker.

12. In an electronic circuit for a currency detector which responds to voltage variations due to an authentic bill to fire a thyratron, which subsequently de-energizes said thyratron, and which thereafter responds to further voltage variations due to said bill to again fire said thyratron, the improvement which comprises a resonant circuit that receives voltage variations generated in said mag netic head and that is connected to the input of said thyratron, and a source of driving voltage for said thyratron, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can provide a cumulative voltage growth capable of firing said thyratron, a circuit breaker that is adapted to normally connect the plate of said thyratron to said source of driving voltage, said circuit breaker responding to the firing of said thyratron to quench said thyratron, and a time delay element that holds said circuit breaker open for a short period of time, said resonant circuit and said time delay element coacting to provide a predetermined minimum length of time between the receipt of the first voltage variation and the re-closing of said circuit breaker.

13. In an electronic circuit for a currency detector which responds to voltage variations due to an authentic bill to produce a first controlled cumulative voltage growth to actuate a threshold device, which subsequently de-energizes said threshold device, and which thereafter responds to further voltage variations due to said bill to produce a second controlled cumulative voltage growth to again actuate said threshold device, the improvement which comprises a resonant circuit that receives voltage variations and that is connected to the input of said threshold device, a circuit breaker that is adapted to normally connect said threshold device to a DC. driving voltage, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can actuate said threshold device,

1 1 said DJC. driving voltage providing a substantially constant driving voltage for said threshold device and thereby providing actuation of said threshold device by the first said controlled cumulative voltage growth as soon as said growth attains a predetermined value and also providing actuation of said threshold device again by said second controlled cumulative voltage growth as soon as said second growth attains said predetermined value, said circuit breaker responding to the actuation of said threshold device to disconnect said threshold device from said D.C. driving voltage, said resonant circuit and said circuit breaker coacting to provide a predetermined minimum length of time between the receipt of the first voltage variation and the re-closing of said circuit breaker.

14. In an electronic circuit for a currency detector which responds to voltage'variations due to an authentic bill to produce a first controlled cumulative voltage growth to actuate a threshold device, which subsequently de-energizes said threshold device, and which thereafter responds to further voltage variations due to said bill to produce a second controlled cumulative voltage growth to again actuate said threshold device, the improvement which comprises a resonant circuit that receives voltage variations and that is connected to the input of said threshold device, a DC. driving voltage for said threshold device, said resonant circuit requiring a predetermined minimum number of said voltage variations at its resonant frequency before it can actuate said threshold device, said D.C. driving voltage providing a substantially constant driving voltage for said threshold device and thereby providing actuation of said threshold device by the first said controlled cumulative voltage growth as soon as said growth attains a predetermined value and also providing actuation of said threshold device again by said second controlled cumulative voltage growth as soon as said second growth attains said predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN AN ELECTRONIC CIRCUIT FOR A CURRENCY DETECTOR WHICH RESPONDS TO A PREDETERMINED MINIMUM NUMBER OF VOLTAGE VARIATIONS, THAT ARE GENERATED AS ONE SIDE OF THE PORTRAIT BACKGROUND OF AN AUTHENTIC BILL IS MOVED PAST THE AIR GAP OF A MAGNETIC HEAD, TO PRODUCE A FIRST CONTROLLED CUMULATIVE VOLTAGE GROWTH TO FIRE A THYRATRON, WHICH SUBSEQUENTLY RESPONDS TO THE MOVEMENT OF THE PORTRAIT OF SAID BILL PAST SAID AIR GAP OF SAID MAGNETIC HEAD TO DE-ENERGIZE SAID THYRATRON, AND WHICH THEREAFTER RESPONDS TO A SECOND PREDETERMINED MINIMUM NUMBER OF VOLTAGE VARIATIONS, THAT ARE GENERATED AS THE OTHER SIDE OF THE PORTRAIT BACKGROUND OF SAID BILL IS MOVED PAST SAID AIR GAP OF SAID MAGNETIC HEAD, TO PRODUCE A SECOND CONTROLLED CUMULATIVE VOLTAGE GROWTH TO AGAIN FIRE SAID THYRATRON, THE IMPROVEMENT WHICH COMPRISES A RESONANT CIRCUIT THAT RECEIVES VOLTAGE VARIATIONS GENERATED IN SAID MAGNETIC HEAD AND THAT IS CONNECTED TO THE INPUT OF SAID THYRATRON, AND A RELAY COIL AND A CIRCUIT BREAKER THAT ARE CONNECTED IN SERIES AND THAT ARE ADAPTED TO NORMALLY CONNECT THE PLATE OF SAID THYRATRON TO A SOURCE OF D.C. DRIVING VOLTAGE, SAID RESONANT CIRCUIT REQUIRING A PREDETERMINED MINIMUM NUMBER OF SAID VOLTAGE VARIATIONS AT ITS RESONANT FREQUENCY BEFORE IT CAN PROVIDE A CUMULATIVE VOLTAGE GROWTH CAPABLE OF FIRING SAID THYRATRON, SAID D.C. DRIVING VOLTAGE PROVIDING A SUBSTANTIALLY CONSTANT DRIVING VOLTAGE FOR SAID THYRATRON AND THEREBY PROVIDING FIRING OF SAID THYRATRON BY THE FIRST SAID CONTROLLED CUMULATIVE VOLTAGE GROWTH AS SOON AS SAID GROWTH ATTAINS A PREDETERMINED VALUE AND ALSO PROVIDING FIRING OF SAID THYRATRON AGAIN BY SAID SECOND CONTROLLED CUMULATIVE VOLTAGE GROWTH AS SOON AS SAID SECOND GROWTH ATTAINS SAID PREDETERMINED VALUE, SAID RELAY COIL RESPONDING TO THE FIRING OF SAID THYRATRON TO OPEN SAID CIRCUIT BREAKER AND THEREBY DE-ENERGIZE SAID RELAY COIL AND QUENCH SAID THYRATRON, AND A CAPACITOR THAT IS CONNECTED IN PARALLEL WITH SAID RELAY COIL TO MAINTAIN SAID RELAY COIL ENERGIZED FOR A SHORT PERIOD OF TIME AND THEREBY DELAY THE RE-CLOSING OF SAID CIRCUIT BREAKER, SAID PERIOD OF TIME BEING COMPARABLE TO THE TIME REQUIRED TO MOVE SAID PORTRAIT OF SAID BILL PAST SAID AIR GAP OF SAID MAGNETIC HEAD AND THEREBY PROVIDE A BLANKING ACTION WHILE SAID PORTRAIT OF SAID BILL IS BEING MOVED PAST SAID AIR GAP OF SAID MAGNETIC HEAD, SAID RESONANT CIRCUIT AND SAID CAPACITOR COACTING TO PERMIT SAID THYRATRON TO FIRE JUST ONCE PER SIDE OF SAID PORTRAIT BACKGROUND. 